BREATHTAKING
images from the Hubble Space Telescope, with its 2.5-meter mirror
lens, have delighted astronomers and the public for years. Now,
the National Aeronautics and Space Administration (NASA) has announced
plans for a progression of larger telescopes to be fielded in space
over the next two decades. These include telescopes with primary
optics whose apertures are 25 meters and more. The increased sensitivity
and resolution of the giant space telescopes will allow astronomers
to view extremely fine features on planets and their moons in our
solar system, image the cores of distant galaxies, and probe the
edges of the universe.The
history of astronomy is dominated by the quest for larger and higher
quality telescopes, says Livermore physicist Rod Hyde. He
notes, however, that using a giant optic in space raises this quandary:
how to design large-aperture space optics that are both optically
precise and can meet the size and weight requirements practical
for launch and deployment. Either of these challenges is,
by itself, quite formidable; in concert, they have yet to be solved,
he says.Hyde
heads a Livermore team that has developed a radically new concept
to overcome the difficulties inherent in building and fielding a
high-quality space telescope far larger than ever deployed. The
concept, called Eyeglass, uses diffractive optics (also called Fresnel
lenses) instead of mirrors or conventional glass lenses.A Fresnel
lens is flat on one side and ridged on the other. It replaces the
curved surface of a conventional lens with many concentric grooves
that are etched into a thin sheet of glass, silica, or plastic to
bend and focus light. Relatively crude Fresnel lenses are commonly
found in traffic signal lights, vehicle headlights, and the rear
windows of motor homes.

In contrast to common mirror
lenses, transmissive diffractive optics are relatively insensitive
to surface imperfections such as bumps and ripples. Because
mirrors reflect light, surface ripples double the magnitude
of the bump. Light passing through a ripple on the surface of
a thin glass or plastic diffractive optic experiences the same
path as light passing next to the ripple, thereby minimizing
any distortion.

Neatly Packaged, Easily FieldedNot only is the Eyeglass diffractive telescope
lightweight, but it also is flexible and can be segmented and folded
into a neat package that fits in a space launch vehicle, says Hyde.
Eyeglass would be easy to field in space because as a thin, flat
membrane, it would not need large, heavy backings, trusses, or motors
to maintain its shape, as do telescopes using mirrors. Conventional glass
lenses and mirrors are far too thick and heavy for large-aperture
space optics, Hyde says. Diffractive optics would make
an ideal lens in space; they would revolutionize deep-space astronomy.Hyde conceived the approach
of using diffractive lenses for large-aperture space optics in 1996.
Since then, the concept has been studied under Laboratory Directed
Research and Development funding and, more recently, with support
from federal agencies. About eight researchers were assigned to
the project from Livermores National Ignition Facility (NIF),
Chemistry and Materials Science, Engineering, and Physics and Advanced
Technologies directorates.The project takes advantage
of long-standing Livermore experience in manufacturing diffractive
glass optics for high-power laser systems such as the Petawatt (see
S&TR, March
2000, The Amazing
Power of the Petawatt) and NIF, currently under construction
at Livermore. NIF will use nearly 1,000 diffractive optics components,
mostly of 40-centimeter-diameter size. A significant number of the
components are being manufactured at Livermore, which has the only
facility in the world that can make precision diffractive optics
of more than a few centimeters in diameter.Diffractive optics can be
made so that they either reflect light (like a mirror) or transmit
it. Mirrors pose serious disadvantages because they are extraordinarily
sensitive to the slightest bump or ripple on their polished surfaces.
A diffractive optic that transmits light, however, is not severely
distorted by surface ripples produced during its operation. Light
passing through a surface ripple experiences the same optical path
as light passing next to the ripple, thereby virtually eliminating
distortion. And by making the diffractive optic slowthat is,
by focusing the incident light farther away from the opticits
surface ripple tolerance can be made up to 100,000 times greater
than for mirrors.

The Livermore team found in
origami, the ancient Japanese art of paper folding, a practical
way to fold and store a lens made of many segments. The team
identified and then simulated several folding patterns for lenses
of various sizes.

No
Motors RequiredMirrors also commonly require
a stiff external skeleton or small motors to maintain their precise
shape to within a few tenths of a nanometer. Such ancillary systems,
which increase weight and complexity, are unnecessary in transmissive
diffractive optics.Furthermore, transmissive
diffractive lenses are themselves more lightweight. Compared to
traditional lenses, the amount of optical material that is required
to focus light with a diffractive lens is quite small. For example,
Hubbles 2.5-meter mirror weighs 800 kilograms. A 25-meter
mirror made more lightweight by removing all unnecessary bulk would
still weigh 7,000 kilograms, far too bulky and heavy to be launched.
Likewise, a 25-meter traditional glass lens would probably measure
6 centimeters thick and weigh about 45,000 kilograms. In comparison,
a 25-meter diffractive lens made of 10-micrometer-thick plastic
would weigh only 10 kilograms.One of the challenges of
fielding a large space telescope is finding a method for stowing
it in a space launch vehicle whose diameter is smaller than the
lenss. The Livermore team has found in origami, the ancient
Japanese art of paper folding, a promising approach to temporarily
contract a lens made of many repeating segments. The principles
of origami are commonly used for map folding as well as product
packaging. The team has worked with origami expert Robert Lang to
identify and then simulate several folding patterns for lenses of
various sizes, including a 5-meter lens. The sequences necessary
to compactly fold lenses of many segments have proved workable in
prototypes using plastic and glass panels.Its difficult
to fold something that is curved, like a mirror. Its much
easier to fold something that is flat, like a diffractive lens,
especially one that is made of many flat segments, says Hyde.
He cites concerns about whether a lens made of many fragile glass
segments can survive the severe vibrations that are associated with
launch. The best approach appears to be to separate the panels with
soft, disposable packing material so that the panels dont
touch one another and then to pack the assemblage tightly.

Early in 2002, the team, guided
by computer simulations, assembled a two-thirds scale model
of a 5-meter lens using unpolished and unetched plastic panels
and successfully demonstrated the origami-like folding pattern.
The folding process used strings attached from an overhead structure
and secured to individual panels. Four of the steps of the folding
process are depicted here. The final step (not shown) was folding
the lens into a configuration measuring 1.2 meters in diameter
and about 55 centimeters high.

A
Color-Corrected TelescopeThe
team has been building and testing increasingly advanced diffractive
lenses with materials that are considered suitable for space missions.
They started by defining the requirements for a space mission, selecting
and characterizing the best materials to make a diffractive lens,
and developing fabrication technologies. Then they built a series
of progressively larger diffractive telescopes and demonstrated
a way to correct for chromatic (color) aberrations.One of the great challenges
of making diffractive lenses suitable for astronomical imaging,
says Hyde, is that a diffractive Fresnel lens focuses different
wavelengths of light at different points in space, thereby distorting
the color characteristics of the image. Because of this effect,
diffractive lenses are mostly used for applications needing only
one wavelengtha monochromatic applicationsuch as for
lasers. In principle, chromatic aberrations can be eliminated by
using a relay lens to reimage an object from the first diffractive
lens onto a second diffractive lens, or inverse Fresnel lens, which
then corrects the aberrations.In 1999, the team developed
a color-corrective optic and incorporated it into the first large-aperture
diffractive telescope. The primary Fresnel lens was 20 centimeters
in diameter and had a focal length of 20 meters. The lens was fabricated
by a photolithographic process that etched a series of diffractive
grooves into 10-millimeter-thick glass. The chromatic correction
system included a 4-centimeter relay lens and a 2.2-centimeter inverse
diffractive lens. The team demonstrated the color correction function
of the system by bringing broadband light (from 470 to 700 nanometers)
to a common focus. Without the correction system, numerous focal
spots generated by the primary lens would span a 7-meter distance.The team then used the telescope
to obtain full-color images of the lunar surface, solar flares,
Jupiter, and Saturn. This telescope successfully demonstrated
that diffractive lenses can be used for imaging over more than an
extremely narrow bandwidth, says Hyde. Four years ago, Eyeglass
received its first external funding, which was used to construct
a 50-centimeter-diameter, color-corrected, f/100 (lens aperture
setting) diffractive telescope. The relatively large diameter and
slow f-number of this lens produced a 50-meter-long telescope. The
team used the laser bay of the Laboratorys now-disassembled
Nova laser to provide a large, vibrationally and environmentally
controlled beam path, which is needed for optically testing the
telescope.

In 1999, the team developed
a color-corrective optic and incorporated it into the first
large diffractive telescope. This telescope, which measured
20 centimeters in diameter and
20 meters long, successfully imaged the lunar surface, sunspots,
solar flares, Jupiter,
and Saturn.

First
Segmented LensSatisfied that they could
manufacture diffractive telescopes capable of operating over all
the wavelengths of visible light, the team began work on overcoming
the packaging challenge for deploying a diffractive lens in space.
Livermore physicist Sham Dixit, who oversaw fabrication and assembly
of the Eyeglass lenses, notes that fabricating a single precision
diffractive optic of 5 meters, let alone one measuring 25 meters,
is far beyond current capabilities. However, even if the team could
manufacture a 25-meter piece of glass, it could never be stowed
in a spacecraft and launched into space. As a result, the Livermore
team focused its efforts on designs that stitch many individual
pieces into one large lens.Dixit says the multipanel
approach is attractive because it splits the fabrication task into
two efforts: optical engineering for creating many meter-scale lens
panels and mechanical engineering for precisely aligning and joining
the panels. The use of multiple panels also provides a practical
way to fold the lens because all folding occurs at metal joints
connecting the flat panels. The joint has to fold, but the
panels do not, says Dixit.In 2001, in an attempt to
demonstrate the feasibility of the multipanel approach, the team
built its first segmented lens. The lens measured 75 centimeters
in diameter and was assembled from six panels precisely aligned
and joined to each other. In optical tests, the lens produced a
tightly focused spot. Following this demonstration, the team folded
the lens into the shape of a piece of pie, unfolded it into a flat
lens again, and observed that the focal spot did not degrade from
the foldingunfolding operation. We achieved our goal
of demonstrating that high-quality, thin, segmented diffractive
lenses could be built with sufficient alignment and seaming accuracy,
says Dixit. Hyde acknowledges some disadvantages
to making a large lens from smaller pieces. The 2- to 4-centimeter
gaps between the segments scatter a small amount of light that could
obscure tiny details, for example, during an attempt to detect a
planet rotating around a much brighter star. Also, the metal seams
holding the panels together expand at a different rate than glass,
thereby causing a small amount of distortion at the panels
edges. Nevertheless, Hyde says, the advantages of a design of multiple
segments far outweigh the disadvantages.Last year, the team began
work to produce 72 glass panels and precisely assemble them into
a 4.7-meter diffractive lens that could be compactly packaged and
deployed in space to meet the space and weight requirements of NASA
and other federal agencies. Our objective was to fabricate
a diffractive lens that is lightweight, foldable, of high resolution,
and that can be scaled up for larger space-based lenses, says
Dixit.

In 2001, the team built its
first segmented lens. The lens measured 75 centimeters in diameter
and was assembled from six foldable panels precisely aligned
and joined to each other.

The final 5-meter lens is
composed of
72 segments: 16 rectangles measuring
654 by 790 millimeters, 32 right triangles measuring 327 by
790 millimeters, and 24 isosceles triangles measuring 654 by
790 millimeters. The panels are divided into eight petals
consisting of three isoceles triangles, four right triangles,
and two rectangles. Each petal covers 45 degrees, or one-eighth
of 360 degrees. One of the petals is highlighted. The circular
lines suggest some of the 19,105 circular etched grooves that
focus the light.

Panels
Polished and EtchedTo make the individual lens
panels, the team started with sheets of commercial zinc borosilicate
glass measuring 1,150 by 850 by 0.7 millimeters. This type of glass
was selected because it is not expensive and is widely used in laptop
computer displays and microscope slides. Forty 700- by 800-millimeter
panels were required for fabricating the 72 panels.The glass sheets contained
several micrometer-deep ripples; they needed to be smoothed to a
flatness within about 0.1 micrometer to obtain the required optical
quality. Because traditional grind-and-polish techniques are expensive
and become increasingly risky for thinner and thinner sheets of
glass, the team explored other methods. The most promising approach
was a wet-etching method developed by Livermore scientists Jerry
Britten and Mike Rushford. They polished thin glass sheets using
a controlled application of acid etchant. This technique polishes
the glass without stressing it. In 2001, the team demonstrated the
effectiveness of this process and built a machine for smoothing
glass sheets.The thin glass sheets were
inscribed with a precise pattern of 0.5-micrometer-deep grooves.
To inscribe the grooves, the team used photolithographic surface-patterning
methods similar to those used in the semiconductor industry. A coating
technique, developed at Livermore, laid down a precise thickness
of liquid photoresist on the lens surface, and an optical pattern
was illuminated through a mask onto the photoresist.All told, the 72 panels contain
19,105 circular grooves. The grooves, about 0.5 micrometer deep,
range from 60 micrometers to several millimeters wide. The grooves
are arrayed concentrically, starting from the centermost panels
and continuing to the perimeter of the outermost panels. The concentration
of grooves ranges from about 1 line per centimeter at the very center
of the assembled lens to about 16 lines per millimeter at the outer
edge.

Magnifying
Glass and Eyepiece at Work in Space

In high Earth orbit, the Livermore-conceived Eyeglass
diffractive telescope would consist of two spacecraft:
a 25-meter-aperture Magnifying Glass and a 1-meter-aperture
Eyepiece. Two vehicles are required because of the Eyeglass
telescopes large aperture and optical slowness.
That aperture and optical combination confers large manufacturing
tolerances but also dictates a focal length of about 1
kilometer. Such a length is impractical for a single spacecraft,
so the Eyeglass telescope would be split into two separate
but cooperating vehicles.
Under this arrangement, the Magnifying Glass vehicle holds
the large-aperture diffractive lens and, with the aid
of a gyrowheel, swivels the lens to point toward desired
targets. The Magnifying Glass gathers and focuses light
to a spot about 1 kilometer away, where the light is collected
by the mobile Eyepiece. The compact Eyepiece also performs
the color correction necessary to obtain accurate images
in visible light of all wavelengths. The two separate
vehicles must remain properly aligned so that they function
together as a high-precision, steerable telescope,
says Livermore physicist Rod Hyde, creator of the Eyeglass
concept.

Hyde says that
after being deployed in space, the giant lens would be
kept flat by being held in tension by rotating the lens
along its axis at about 10 revolutions per minute. However,
spinning would make it harder to swivel the Magnifying
Glass so it could image another target. Hyde solves this
problem by placing a counter-rotating gyrowheel inside
the center of the lens. Although the gyrowheel would replace
the centermost glass panels, these make up only a small
portion of the lens and are not essential. While in space,
the thin glass panels must withstand exposure to meteoroids
and vacuum, sunlight, and radiation. Fortunately, glass
holds up well to the extreme conditions of space. Hyde
calculates that based on data recorded by existing satellites,
a 10-year exposure in space would result in damaging one
ten-thousandth of the Eyeglass lens surface. Impact from
meteoroids would likely create either craters or holes
located about every 2.5 centimeters and many accompanying
cracks. Fortunately, the cracks would not grow because
the lens would be under low tension. Also, the lack of
water vapor in space makes glass much more resistant to
spreading cracks.

The
Eyeglass diffractive telescope would consist of
two spacecraft: a 25-meter-aperture Magnifying Glass
and a 1-meter-aperture Eyepiece.

Assembling
the PanelsThe
72 lens panels were cut into precise rectangular and triangular
shapes for assembly into the complete lens. The assembly, done by
a group led by engineer Andrew Weisberg, used the same process demonstrated
on the 75-centimeter lens but upgraded to account for the larger
size, panel count, and tolerance requirements of the 5-meter lens.
Dixit notes that when working on individual panels, one must never
lift them by the edges but rather slide them on a smooth backing,
much like using a pizza paddle.Once a panel was in the proper
location, it was joined to its neighbors by gluing each piece to
foldable metal. Having panels out of register, says Dixit, would
be disastrous to image quality. Precision alignment can be ensured
by matching fiducials (tiny marks) etched along the common borders
of neighboring panels to a precision of 1 to 2 micrometers. About
250 micrometers thick, the seams can withstand forces much greater
than those it would likely experience during deployment in space.The assembled 5-meter lens
has a focal length of 250 meters and an optical speed of f/50. Its
72 panels include 16 rectangles measuring 654 by 790 millimeters,
32 right triangles measuring 327 by 790 millimeters, and 24 isosceles
triangles measuring 654 by 790 millimeters. The panels form eight
petals, each consisting of three isoceles triangles,
four right triangles, and two rectangles. Each petal covers 45 degrees,
or one-eighth of 360 degrees.With this configuration of
repeating triangles and rectangles, the entire lens can be folded
in an intricate but foolproof manner and fit into a hatbox measuring
1.75 meters in diameter and about 80 centimeters high. The team
gained confidence in the folding patterns by building subscale models
from plastic and glass panels. Following assembly, the lens
was mounted in a steel frame and a mesh of aluminum bars on each
side to keep the lens rigid for transportation to an outside testing
location and to protect it against winds. Although the team verified
the characteristics of the individual panels during the fabrication
and assembly process, optically testing the complete lens was still
required. Upon delivery at its testing
location, the horizontal lens was lifted by a crane to a vertical
position and then secured. The lens was illuminated at night with
532-nanometer laser light, producing 1- to 2-centimeter-diameter
image spots. Although the optical test was successful, Hyde calls
it a rudimentary test because, as expected, air currents and the
lack of the panels complete flatness caused some distortion
of the focused spot. An ideal testing environment, Hyde says, would
be an underground tunnel at the Department of Energys Nevada
Test Site. Nevertheless, the test at Livermore was considered appropriate
for this first-generation lens.

The completed lens, mounted
in a steel and aluminum frame and ready for optical testing.

(a) The lens during optical
testing. Some of the panels are obstructed by the supporting
frame. (b) The 1- to 2-centimeter focal spot produced by the
lens when illuminated with 532-nanometer laser light.

On
the MapThe
lens is, by a wide margin, the largest optical-quality lens in the
world. For example, it has twice the diameter of the primary mirror
for the Hubble Space Telescope, yet is 10 times lighter.A 5-meter lens is a
big-league optic. Demonstrating such a large Fresnel lens places
diffractive optics firmly on everyones map, says Hyde.
By making the lens from technology that is scalable to much
larger sizes and from space-deployable materials, we have demonstrated
the technology and the here-and-now reality of diffractive telescopes.
A 5-meter diffractive space
telescope could be deployed in space within two to three years,
says Hyde. A 25-meter or larger version could be deployed within
a decade.The team is exploring preliminary
partnerships with U.S. agencies that could benefit from diffractive
telescopes. Discussions have focused on design, technology development,
and demonstrations of lenses of 5 meters and larger. Hyde also plans
to establish partnerships with traditional space contractors. The
Livermore role in these partnerships would be to support the optical
and deployment designs and serve as the fabrication house for the
lenses.One option under exploration
is obtaining even thinner glass sheets to save additional weight.
Another option is fashioning a lens from segments made of polymer
films. A plastic lens would be less prone to damage from launch
vibration, would weigh less, and could be fashioned from multiple
panels that are larger than their glass counterparts. The Livermore
team has carried out research on polymer films and done etching
on several meter-size panels.Hyde adds that the technology
developed at Livermore could be used for more than astronomy. Lightweight
diffractive optics of greater than 10 meters would likely be used
in applications such as Earth observation and optical communications.
Closer to home, Everything were learning about making
diffractive optics benefits the National Ignition Facility and high-powered
lasers everywhere, he says.The Livermore team has put
diffractive telescopes on the map. The next job is putting them
into space.Arnie Heller